Personal computer systems in general and IBM personal computers in particular have attained widespread use for providing computer power to many segments of today's modern society. Personal computer systems can usually be defined as a desk top, floor standing, or portable microcomputer that consists of a system unit having a single system processor, a display monitor, a keyboard, one or more diskette drives, a fixed disk storage, and an optional printer. One of the distinguishing characteristics of these systems is the use of a motherboard or system planar to electrically connect these components together. These systems are designed primarily to give independent computing power to a single user and are inexpensively priced for purchase by individuals or small businesses. Examples of such personal computer systems are IBM's PERSONAL COMPUTER AT and IBM's PERSONAL SYSTEM/2 Models 25, 30, 50, 60, 70 and 80.
These systems can be classified into two general families. The first family, usually referred to as Family I Models, use a bus architecture exemplified by the IBM PERSONAL COMPUTER AT and other "IBM compatible" machines. The second family, referred to as Family II Models, use IBM's MICROCHANNEL bus architecture exemplified by IBM's PERSONAL SYSTEM/2 Models 50 through 80.
Beginning with the earliest personal computer system of the family I models, such as the IBM Personal Computer, it was recognized that software compatibility would be of utmost importance. In order to achieve this goal, an insulation layer of system resident code, also known as "microcode", was established between the hardware and software This code provided an operational interface between a user's application program/operating system and the device to relieve the user of the concern about the characteristics of hardware devices. Eventually, the code developed into a BASIC input/output system (BIOS), for allowing new devices to be added to the system, while insulating the application program from the peculiarities of the hardware. The importance of BIOS was immediately evident because it freed a device driver from depending on specific device hardware characteristics while providing the device driver with an intermediate interface to the device. Since BIOS was an integral part of the system and controlled the movement of data in and out of the system processor, it was resident on the system planar and was shipped to the user in a read only memory (ROM). For example, BIOS in the original IBM Personal Computer occupied 8K of ROM resident on the planar board.
As new models of the personal computer family were introduced, BIOS had to be updated and expanded to include new hardware and I/O devices. As could be expected, BIOS started to increase in memory size. For example, with the introduction of the IBM PERSONAL COMPUTER AT, BIOS grew to require 32K bytes of ROM.
Today, with the development of new technology, personal computer systems of the Family II models are growing even more sophisticated and are being made available to consumers more frequently. Since the technology is rapidly changing and new I/O devices are being added to the personal computer systems, modification to the BIOS has become a significant problem in the development cycle of the personal computer system.
For instance, with the introduction of the IBM Personal System/2 with MICROCHANNEL architecture, a significantly new BIOS, known as advanced BIOS, or ABIOS, was developed. However, to maintain software compatibility, BIOS from the Family I models had to be included in the Family II models. The Family I BIOS became known as Compatibility BIOS or CBIOS. However, as previously explained with respect to the IBM PERSONAL COMPUTER AT, only 32K bytes of ROM were resident on the planar board. Fortunately, the system could be expanded to 96K bytes of ROM. Unfortunately, because of system constraints, this turned out to be the maximum capacity available for BIOS. Luckily, even with the addition of ABIOS, ABIOS and CBIOS could still squeeze into 96K of ROM. However, only a small percentage of the 96K ROM area remained available for expansion. With the addition of future I/O devices, CBIOS and ABIOS will eventually run out of ROM space. Thus, new I/O technology will not be able to be easily integrated within CBIOS and ABIOS.
Due to these problems, plus the desire to make modifications in Family II BIOS as late as possible in the development cycle, it became necessary to off load portions of BIOS from the ROM. This was accomplished by storing portions of BIOS on a mass storage device such as a fixed disk. Since a disk provides writing as well as reading capabilities, it became feasible to modify the actual BIOS code on the disk. The disk, while providing a fast and efficient way to store BIOS code, nevertheless greatly increased the probability of the BIOS code being corrupted. Since BIOS is an integral part of the operating system, a corrupt BIOS could lead to devastating results and in many cases to complete failure and non-operation of the system. Thus, it became quite apparent that a means for preventing unauthorized modification of the BIOS code on the fixed disk was highly desireable.